This invention relates to a pressure sensor and more particularly, a pressure sensor which relies on changes in capacitance to indicate pressure fluctuations.
Capacitive pressure sensors are well known in the prior art. Such sensors typically include a fixed element having a rigid, planar conductive surface forming one plate of a substantially parallel plate capacitor. A deformable conductive member, such as a metal foil diaphragm, forms the other plate of the capacitor. Generally, the diaphragm is edge-supported so that a central portion is substantially parallel to and opposite the fixed plate. Since the sensor generally has the form of a parallel plate capacitor, the characteristic capacitance of the sensor is inversely proportional to the gap, d, between central portion of the diaphragm and the conductive surface of the fixed element. In order for there to permit a pressure differential across the diaphragm, the region on one side of the diaphragm is sealed from the region on the opposite side.
In practice, the diaphragm elasticity is selected so that pressure differentials across the diaphragm in a particular range of interest cause displacements of the central portion of the diaphragm. These pressure differential-induced displacements result in corresponding variations in the gap, d, between the two capacitor places, and thus in capacitance variations in the sensor capacitor. For relatively high sensitivity, such sensors require high changes of capacitance in response to relatively small gap changes. To achieve such sensitivity from unit to unit, nominal gap dimensions generally require that their component parts be manufactured to very close tolerances to establish the required dimensional relationships. In addition, the structure and materials must maintain those relationships over a useful temperature range.
In one form the prior art sensor, exemplified by the sensor of the Model 237 transducer manufactured by Setra Systems, Inc, assignee of the subject invention, a fixed metallic electrode is supported with respect to the diaphragm support member by means of an electrically non-conductive glass portion. Because of the differences in the thermal expansion coefficients between such glass and the metal electrode and diaphragm support member, temperature changes cause changes in the gap between the fixed electrode and diaphragm (i.e. the plates of the capacitor), resulting in erroneous pressure readings. In addition, the effective seal between the regions on opposite sides of the diaphragm may only be economically maintained over a relatively small temperature range. Thus, such sensors give reliable pressure readings over only a relatively small range of temperatures.
Moreover, during the manufacture of such sensors, the fixed electrode must be set in the glass portion and then the conductive plate portion must generally be tailored (e.g. by lapping) to establish the necessary gap and parallelism. Consequently, those variable capacitance pressure sensors are relatively expensive to manufacture.
Moreover, during the manufacture of such sensors, the fixed electrode is generally set into the glass portion while the glass is in its molten state. As the assembly cools, mechanical stresses are set up which typically alter the desired initial gap dimension or degrade the parallelism between the capacitive plates. Following cooling of the assembly, the fixed electrode may need to be tailored (e.g. by lapping) to re-establish the critical gap and parallelism. In view of these processing steps, such sensors are relatively difficult and correspondingly expensive to manufacture.
Another type of prior art sensor is disclosed in U.S. Pat. No. 4,358,814, assigned to the assignee of the subject invention. That prior art sensor, exemplified by the Models 264 and C264 transducer and transmitter, manufactured by Setra Systems, Inc., includes a cup-like, or concave, metal base member coupled to a base support at the center of the bottom of the base member. The base member includes a peripheral flange portion extending from its rim, where the flange is generally planar except for a circumferential depression. A relatively thin, deformable conductive diaphragm is disposed across the peripheral flange of the base member. A clamping ring having a surface which complements the flange of the base member is affixed to the edge of the diaphragm and the flange so that the diaphragm is clamped under tension to the flange.
An electrode assembly is affixed to the base support within the closed volume formed by the base member and diaphragm assembly. The electrode assembly includes a conductive electrode having a planar portion and a dielectric support member. The support member is connected to the base support so that the planar portion of the electrode is substantially parallel to and displaced by a predetermined distance d, from the flange of the base member. With this configuration, the diaphragm and the planar portion of the electrode form a parallel plate capacitor. Moreover, in response to pressure differentials, the diaphragm is displaced, resulting in corresponding changes in capacitance. Electrical connection to the capacitor may be provided by direct connection to the base member for one plate and a feed-through connection to the plate formed by the planar portion of the electrode.
The latter type of prior art sensor is quite effective in the measurement of pressure. Moreover, there are few high tolerance parts and complex assembly operations, with the only critical dimensional assembly operation being the initial alignment of the electrode with respect to the flange of the base member. Since this step only involves solid materials at room temperature, there are minimal mechanical stresses established. Moreover, since there are no glass-to-metal seals, there are no problems due to mis-match of temperature coefficients. Consequently, that prior art sensor provides a high sensitivity broad temperature range capacitance pressure sensor.
However, the latter type of prior art sensor has not been practical in certain segments of the market due to its relative complexity and relative high cost of manufacturing. Part of the high manufacturing cost is because of the necessity of controlling the thickness of the metallized electrode of that sensor and the dimensional tolerance of the main housing. The metallized electrode, which is generally made of ceramic disc, also represents significant cost. In addition, there is a need of an outside casing to house the sensor and the measurement circuit.
It is an aspect of the present invention to provide an improved pressure sensor.
Another object is to provide a high performance pressure sensor that is relatively inexpensive and easy to manufacture.